Loop Variable Not Used in Loop Body

Severity: HIGH — D95237240 caused animation glitches in variable frame rate GIFs

What to Look For

• for or while loops where the declared iterator variable is not referenced in the loop body
• Method parameters with similar semantic meaning to the loop variable (e.g., frameNumber vs i)
• Array/list indexing that uses a parameter instead of the loop iterator

When to Flag

• Loop declares variable i or similar, but loop body uses outer scope variable instead
• Pattern: for (i in 0 until n) { array[n] } — uses n instead of i
• The loop would produce incorrect results (repeating same value N times)

Do NOT Flag

• Loops where the iterator is intentionally unused (e.g., repeat N times)
• Loops with explicit @Suppress("UNUSED_VARIABLE") annotation
• Loops where the iterator is used in a nested scope

Examples

// BAD — uses parameter frameNumber instead of loop variable i
for (i in 0 until frameNumber) {
    sum += frameDurations[frameNumber]  // ❌ Wrong: adds same duration N times
}

// GOOD — uses loop variable i correctly
for (i in 0 until frameNumber) {
    sum += frameDurations[i]  // ✅ Correct: sums durations 0 to frameNumber-1
}

Recommendation

When writing loops that iterate through a range, always verify the loop body uses the iterator variable for indexing, not the range boundary parameter.

Evidence

• D95237240: getTargetRenderTimeMs() in DropFramesFrameScheduler used frameNumber instead of i, causing incorrect animation timing for GIFs with variable frame durations
• Impact: jumpToFrame() calculated wrong loop counts, causing premature animation termination